In the year 2116 C.E. scientist can easily make multiple clones of a human, I was thinking maybe the doctor can make some markings on the clone body however what happens if the doctor forget to tag. Answer with the most reliable method of identification that can see through the most forgery wins.

$\begingroup$Isn't the whole point of a clone to make it impossible to tell the two apart? A perfect clone, by definition, cannot be told apart from the original...$\endgroup$
– AifySep 22 '16 at 6:09

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$\begingroup$Then what you need isn't a way to tell the clones apart, you need a flawed cloning system, which could be whatever you want it to be; eg: opinion/story based.$\endgroup$
– AifySep 22 '16 at 7:13

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$\begingroup$I notice from the answers you got so far, that there are two different interpretations of your question: One: "If you see/examine someone, is there a test telling whether he is the result of cloning?" Other: "If you see/examine both an "original" human and his clone, how do you tell which one is the original?" These are two very different questions, Which of them did you mean?$\endgroup$
– celtschkSep 22 '16 at 8:18

$\begingroup$Also, what kind of cloning are we talking about? The biological kind leaves plenty of room for identification in the form of processes which are not genetic. However, if you have a "magical" cloning box which makes a physically identical body and identical mind, then we may have to explore ways to tweak that magic to make identification easier. That is, of course, presuming identification is important at all. Many cloning stories point out that the lines between individuals get muddy with such technology, and unique identification may not as much value as you think.$\endgroup$
– Cort AmmonSep 22 '16 at 13:55

17 Answers
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The simplest means, which doesn't require any genetic testing, is to look for signs of wear and tear. Scars on the hands, the thicker places on bones where they have been broken and healed, which show up on X-rays, fillings in teeth - none of these things are genetically determined.

Fingerprints won't be identical either, since they are different between identical twins.

If you can get a little fancier, antibodies to diseases that one body has had, and another has not, will also work.

$\begingroup$And their related cousin, retina scans. The retina scans are even harder to fudge, though obviously they're also harder to use in the first place.$\endgroup$
– Cort AmmonSep 22 '16 at 14:14

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$\begingroup$I'd guess that even if you were to create an identical-to-the-atom copy of a living organism, the moment they start "living" on their own, signs of wear and tear would differ.$\endgroup$
– OskuroSep 22 '16 at 16:40

$\begingroup$Fingerprints are only partially genetic. The rest is due to stresses in the womb and stuff that happens after birth (cuts, for example).$\endgroup$
– jaxad0127Sep 22 '16 at 20:08

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$\begingroup$@Oskuro An "identical-to-the-atom copy of a living organism" isn't a clone, it's a duplicate. As John Dallman said, there's "an established biological meaning" for what is a clone. Atom-by-atom copies aren't clones, they're duplicates. if they were called duplicates instead of clones there wouldn't be an issue.$\endgroup$
– a4androidSep 24 '16 at 2:12

Scars

As you grow up you hurt yourself. If you did not then I pity you for having an extremely boring childhood.

...or you get Chickenpox...

...or you get acne and just cannot keep from poking at the scabs...

...or you...

...well you get the point: the human body is subject to wear and tear.

The clone would not have these telltale signs of actually having lived a long and full life.

If you want a more "nerdy" method: telomers. These tags at the end of our chromosomes are a very clear indication of how old different organs "really" are. If you are cloning someone, unless you also clone the exact state of the telomers, then you should be able to tell the clone apart from the original by the fact that the telomers of different organs are the "wrong" length for a person of that age.

EDIT: Stealing shamelessly from John Dallman below (please up-vote his answer because it is great)...

Fingerprints, not even identical twins get identical fingerprints. The same would go for other biological distinguishers, like iris pattern. Although you need to have a print of the original before you try to compare.

Antibodies, the immune system contains a historical record of what illnesses you have had, illnesses the clone have not had.

$\begingroup$did you really needed to take stuff from his answer?$\endgroup$
– MindwinSep 22 '16 at 13:01

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$\begingroup$@Mindwin I acknowledged as much and encourage people to vote for it. I honestly think his answer will benefit more from that than the other way around.$\endgroup$
– MichaelKSep 22 '16 at 13:02

$\begingroup$@Mindwin Given that this is currently the topmost answer and some visitors may not read all of the answers they might miss these methods on the original answer if they aren't stated here as well.$\endgroup$
– AnnonymusSep 22 '16 at 14:10

$\begingroup$@Annonymus taking it to Meta. Let the community decide what the best course of action is.$\endgroup$
– MindwinSep 22 '16 at 18:00

There is a simple way to tell any two clones apart. You just ask them which clone are you?

Forget about fantasy clones where clones are the exact duplicates of their original, real clones will be no different from identical twins. They won't emerge from a vat as a carbon copy of the person copied. Clones will grow in the normal way ordinary human beings are grown. They will have memories of their lives and their experiences. Clones may be genetically identical, but they will be distinctly different human beings. Each with their own identities, individual minds and personalities.

If the problem is distinguishing between identical human beings, then they won't be clones they will be duplicates of human beings. However, that duplication process worked the one thing that is certain is that whatever the process is, it won't be cloning.

Telling the difference between any two duplicated human beings is an entirely different problem from distinguishing any two clones. Clones are easy, duplicates are hard in principle. However, in practice, the answer is easy just microchip them. By 2116 CE, microchipping should have improved out of all sight. Advances in nanotechnology, communications technology, and organic electronics to name a few. Microchipped duplicates, easy as that.

$\begingroup$Clones will be as truthful as normal human beings. It takes more than one question to find the right answer. Real clones, not fantasy clones, will usually know they are clones. Although like adopted children they might be told about their origins. Duplicated people may be different, but that depends on the duplication process. Kindly step into the matter duplicator, @celtschk, your number is about to go up.$\endgroup$
– a4androidSep 22 '16 at 8:41

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$\begingroup$"Clones will be as truthful as normal human beings." — That's the question. As far as I can tell, for any mammal clones we've done so far, the clone was not indistinguishable from a normal mammal. See for example the second link in my answer.$\endgroup$
– celtschkSep 22 '16 at 9:49

$\begingroup$@celtschk I agree. Clones won't be indistinguishable. The trouble is the myth is that they are indistinguishable and identical. This would only be so for duplicates, and clones aren't that.$\endgroup$
– a4androidSep 22 '16 at 12:58

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$\begingroup$As an identical twin, I claim to be an authoritative source on this topic, and I strongly support this answer :-) :-) While I am an exact clone of my ... clone ... we only share genetics. We still have differences in fingerprints, retinas, facial features, health histories, and so on. Still, when asked who's who, we usually first check what weekday it is, because we (claim to) swap identities every other day.$\endgroup$
– Torben Gundtofte-BruunSep 23 '16 at 7:54

This series has so many clones. All are different. Here are some tellers you can use to distinguish all the clones even when they try to impersonate other clones (cheers to Tatiana Maslany for the excellent acting there).

Can you tell the clones apart? I sure can!

Personal preferences

The way they dress is different. Some might prefer a specific style, a color, or even a specific fabric of a cloth (cotton, wool, etc.).

Clones will also likely have different hair length, style or color.

Their make up will also more than likely be different.

Nurture-based tellers

The voice is a... teller (sorry). It is something that is learned. Your throat structure is set but various factors influence your voice: hormones, pollutants, usage of the voice, languages learned (since they reshape slightly your throat to mimic the language better).

The accent will likely be different as well. If one is from London, I bet the accent will be different from one from New York.

Their scars will tell them aparts. They lived differently and got hit differently.

Behavior

Manners are acquired. The way someone writes, moves, behaves in society is unique. Some might be more shy than others. Some have learned to walk a bit weird.

$\begingroup$Well, she doesn't know what Beth sounds like so she doesn't imitate Beth and Maslany speaks as Sarah when Sarah impersonates Beth. The fact that people who know Beth can't tell her apart from Sarah is a weak point in the season 1, but in that series, who could think that clones do exist? Sarah-as-Beth could say she has a cold, people won't try to check who she really is because they can't think clones exist.$\endgroup$
– Olivier GrégoireSep 23 '16 at 11:56

$\begingroup$The Orphan Black clones are all genetically unique. A quick look at their DNA would be all that would be required to tell them apart. I suspect genetic markers would be placed in the DNA of the future clones, sort of like a bar code. Just sequence their DNA, which would be a quick process, and you will get the answer. Also in the future, DNA testing would be accessible to almost everyone, so it wouldn't be much of a stretch to check the DNA.$\endgroup$
– Jason HutchinsonSep 23 '16 at 13:20

$\begingroup$I'm not answering the "most reliable" method: I only push the fact that you don't have to go that far.$\endgroup$
– Olivier GrégoireSep 23 '16 at 13:22

$\begingroup$Likewise, the moledular biologist would say the same thing but via the phrase "telomere length". Cell division shortens telomeres so the younger clone has longer telomeres.$\endgroup$
– Eric TowersSep 24 '16 at 19:41

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$\begingroup$@EricTowers -- except that the telomeres are likely to be copied from the original person, so the younger clone should have telomeres the same length as the original (but shorter than would be expected for a person of their apparent age -- which would give you a way of identifying a clone even if the original was not available for comparison).$\endgroup$
– Periata BreattaSep 25 '16 at 2:55

I think the one way would be epigenetics. Epigenetics is about information "attached" to the genes which affect the expression (roughly, "execution") of genes.

One of the problems in cloning mammals (the group to which humans belong) is that clones differ in their epigenetic traits from the cloned individuum. Now it is not unreasonable that science manages to get this sufficiently under control that for all intents and purposes the appearance and functionality of the cloned body is the same (or rather, differs no more than to be expected due to different lives afterwards), but not enough that the different imprinting could not be detected with a test.

I feel like an obvious, if boring, answer is that the clone would not have the memories of the original. That can be something overt and obvious, like what colour the original's cat was, or whether or not the original's mother was left handed, or it can be something discreet, like the clone being exposed to some innocuous bit of deliberate disinformation or awkward trivia as they grew-- a weird vocal tic, or having been taught incorrect lyrics to a song, or being absolutely convinced that a tomato is a vegetable and not a fruit.

To that extent, a clone would likely have at least slightly different mannerisms from the original creature. Differences between multiple clones could be made that way too: perhaps each clone is introduced to their creator separately, and in that introduction, he or she identifies themselves by a unique name. The clone who calls the creator "Sasha" is the first one; the one who calls the creator "Yuri" is the second, "Rene" is the third, and so on.

Obviously, the clone that calls the creator "you bastard" is the original.

In any case, although it is not necessarily the most accessible distinction, it would be fairly easy to tell the clones apart by the information they possess, be it as subtle as a weird nervous habit or as exaggerated as teaching them all different languages.

When they created Dolly, the first sheep they cloned, they knew that the clone would have a shorter lifespan because of the normal damage that happens to the DNA over the years. While the Dolly appeared to be younger, she ended up dying of an old sheep's disease.

This has to do with the slow erosion that happens after repeated cell divisions, at least from my understanding.

Assuming cloning goes that approach, there will probably be tell-tale signs of decay in the DNA that would indicate that the clone was older then they actually were. That would make it easier to identify the clone over the original (much line you can tell a car engine has its odometer rolled back by the wear and tear on the parts).

Another approach has to do with memories and experiences. With our current existence of having almost every event in ours lives photographed and documented. And those images stored on a medium that appears not to forget, you could probably identify a clone by a sudden change in appearance (they got a boob job... no, just twenty years younger), a disappearance (it takes nine months to make a clone), or a persistent recall of past events without emotions (e.g., they learned them instead of experienced them, however that wouldn't apply to certain mental issues, so probably not consistent).

That last one is kind of related to chaos theories that you can't have a perfect copy unless you had the starting conditions. That also means that every single event influences a person. Even a couple differences, or a different emotional context, would create a person who would deviate further away from the original with every new occurrence. Each clone would acquire different personality traits over time, spreading apart until they are completely distinct personas days, weeks, or months after the point of cloning.

$\begingroup$As it happens, the apparent premature aging experienced by Dolly appears to have been a coincidence. Her sisters (also cloned) and other cloned animals have not shown this effect. sciencenews.org/article/…$\endgroup$
– user2390246Sep 23 '16 at 13:12

Barcodes

You could conceivably place barcodes on the back of their head or anywhere you want really. Now just get one of those supermarket barcode scanners, link it to the clone database and scan your clone's barcode to find out if he is [User0001] or [User6760]. You can just tattoo the barcode onto them and it should be there pretty much for the rest of their lives unless they tamper with it. And yes, I did steal this from Hitman.

$\begingroup$"however what happens if the doctor forget to tag". OP is not looking for ways to tag a clone but how to tell them apart in case someone forgot to do so, as it says in the question.$\endgroup$
– Broots WaymbSep 22 '16 at 14:14

$\begingroup$Make it an RFID with PayPass that doubles as driving license / some other ID and people will inject it voluntarily$\endgroup$
– DaerdemandtSep 23 '16 at 11:55

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$\begingroup$@Daerdemandt You clearly haven't interacted with the surprisingly large portion of the population that thinks such a tag would be the "Mark of the Beast" from the bible. Now hopefully you can raise your clone not to think that is the case, but if so, then you could definitely mark them in some other way$\endgroup$
– Kevin WellsSep 23 '16 at 18:29

$\begingroup$@KevinWells Most of these people are easy to deal with - there's a system to explain them what they should believe. If their beliefs would be really bad for business, they'll be altered and "Mark of Jesus" would be welcome.$\endgroup$
– DaerdemandtSep 23 '16 at 18:38

The clone would be younger than the original. OP mentions in the comments that we are talking about the "normal" method of cloning.

Assuming you make the clone at a very close date to the original's birthdate, then...

Finger prints (as others have mentioned) are formed while the fetus is in the womb

X-inactivation is something that happens in females (so it won't work for males) where in some cells one of the X chromosomes is active and the other is not and in other places the opposite is true. This is why calico cats have patterns. The pattern of the cells is, like finger prints, determined in the womb. Humans are not patterned like cats because the traits expressed in the chromosome which may be active or inactive doesn't include skin or hair color. However, you still could take samples from various places on the original and compare those to the potential clone.

I can't think of anything else off the top of my head, but anything that is determined in the womb as opposed to genetics would work.

There is an Asimov I, Robot story about a robot that attempts to hide amongst other identical robots. They discover it by establishing that there is a small piece of knowledge that the robot does not possess, and testing each robot in turn.

As I recall, the other robots have had training to understand about detecting electromagnetic waves. They drop a heavy item onto a human that has such waves above their head such that they will be completely unharmed because the object will be deflected harmlessly to one side. As robots are compelled to protect human life at all cost, they would intervene if the human was in danger. Those who have had the training would not react, but the 'spy' robot is unaware that there is no danger and intervenes to save the human from being killed, thus revealing itself.

Another example...

Depending on how the clones memories are handled, there is also an episode of TV show Red Dwarf where a monster takes on the appearance of one of the crew. As a 'clone' it appears 100% indistinguishable, and it scans the mind of the target to gain its complete personality, making it identical in mannerisms etc.

However, the crew member it targets throughout the show has the delusion that he is an amazing guitar player when really he is terrible, and the clone does not realise this is delusion when it scans his mind. When tested, the clone reveals it can play the guitar with incredible skill equal to the crew members belief, revealing itself.

$\begingroup$that Red Dwarf storyline was probably ... err .. borrowed from the story The Brain Stealers of Mars by John W Campbell. Although that's not quite as funny, I guess. :)$\endgroup$
– Periata BreattaSep 25 '16 at 3:10

$\begingroup$Interesting! Most of the episodes are taken from other things, didn't know about that one.$\endgroup$
– SLCSep 26 '16 at 14:56

Assuming a 'perfect clone' with an identical physical composition, rather than a clone created merely by DNA, I am not entirely content with the workaround of wear and tear...

Stealing a clever thought from The Thing, it is likely that a clone would consist solely of biological matter, and thus would lack jewellery, fillings, artifacts from surgery, etc. from the original. While the jewelery is easily fixed after the cloning process, it would require a great deal more effort and ingenuity for a clone to go about getting fillings put in at a dentist, or having a metal plate put in its knee.

So, I believe a check for fillings would be quite reliable for a majority of individuals, while metal plates and the likes would be a more reliable test for a smaller subset of people.

Given that we're talking about the future, maybe fillings no longer exist... however, we could be even more liberal and assume that each individual has a microchip embedded in them which wouldn't be copied in the cloning, or perhaps the medical field has advanced significantly and more people have inorganic insertions in their bodies.

$\begingroup$About 30 minutes after your answer, the OP stated they were talking about genetic cloning, not "magic" cloning. Also, the question is about whether a specific person is a clone, not whether they're the original. That said, I don't understand your logic. First, why would a particle-level clone not have the fillings, etc., of the original? If we're talking about a transporter malfunction or something, it's literally the same person down to the fiberglass irritating their skin from earlier. Second, why would a society with this level of cloning use anything but cloned body parts during surgery?$\endgroup$
– MichaelSSep 25 '16 at 10:45

You take DNA sample frm one being and insert in the seeding ovum. This clone will grow from scratch in vivo.

Therefore the original will be physically older than the clone, but their genetical age will be the same. 1 year later the original will be mid-aged bussinessman and his clone will be tiny model of sir Winston Churchill.

If one can simulate the baby growth during pregnancy, the process can be faster, but the clone will last the signs of the expirience to the world - scars, immunity system "database".

To have original-clone pair that cannot be distinguished one must copy and paste every single cell in the body, every single neural connection, every single bit of information the body had acquired. If any single feature cannot be copied, this feature is the item on the checklist you are looking for.

$\begingroup$Actually, no. To create a clone from a single cell of a multicellular organism, you have to completely reset that cells epigenetic code. In simple language, to remove all its specialisation and return it to the state of a newly fertilised egg cell or one of the first few divisions thereof. The clone may inherit some DNA mutations from its donor cell (meaning it's not a perfect clone, potentially meaning health problems for the clone), but it won't be a baby with tissue as aged as it's donor. Cloned animals have lived as long or longer than the expectation for their breed and species.$\endgroup$
– nigel222Sep 23 '16 at 19:51

$\begingroup$Every age counter will start from zero. Only the DNA will start pre-aged. I think you cannot undo errors created during DNA replication (cell division). The point is that You will be 46, for example, but your clone will be 1 year old.$\endgroup$
– CrowleySep 26 '16 at 12:24

I'm not sure if radiocarbon dating can be used on such small timescales, but the notion would be that, even if the clones are identical at the cellular level (including memories), radioactive elements within the original's body would've decayed/changed naturally, and as such would provide an indicator of real age, as opposed to mental or cellular aging.

If the copy is made to the atomic (or subatomic) level, just have the clones go for a walk, and then differentiate micro lesions from regular activity to identify them (because if your technology is good enough for atomic-level cloning, then it should be able to measure such differences easily), of course this last method won't solve who the original was in the first place.

$\begingroup$This isn't how radiocarbon dating works. The reason it works for fossils and rocks is that the rocks are a closed system that doesn't allow the influx of un-decayed radioactive isotopes, so we can see how long it has been since that system was last open (which is typically how long the rock or fossil has existed). That doesn't work on living things because we are constantly replacing the atoms in our bodies, so we don't have a stable closed system to measure.$\endgroup$
– Kevin WellsSep 23 '16 at 18:27

$\begingroup$That said, a more sensitive variant of a more traditional carbon dating technology might turn out to be useful for near-term testing in the future, and the same technique of testing tooth enamel to determine an approximate year of birth would then be usable with that system.$\endgroup$
– Periata BreattaSep 25 '16 at 3:19

$\begingroup$@PeriataBreatta Fair enough, I didn't realize that tooth enamel was that stable. However, as that article points out, that was still used on human remains, not living humans, presumably because it destroys the tooth enamel for testing (not something you want to make a habit of while alive)$\endgroup$
– Kevin WellsSep 26 '16 at 15:58

$\begingroup$@Kevin Wells I suspected as much, hence my "or similar method" note. My point was really to look down into the atomic level to differentiate a copy from the original.$\endgroup$
– OskuroSep 27 '16 at 8:06

A clone, is hardly a clone without it cloning the wear and tear of a person.. Yes, if they can successfully clone people to a degree in which markings is a necessity - they can replicate all the wear and tear too, even fingerprints.
It would not be a clone without it.

At this point, only a genetic sample to determine the cell age would be viable and this is assuming the clone was not made at the birth of the cloned person.

In fact, if they are so advanced - there probably is no way to tell other than mark them.